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$EDITOR as unix filter

It is really, really strange, but none of major text editors –
neither vim, nor emacs work as filter – I mean, you can’t pipe text
into stdin, edit and have edited version on stdout. That is because
stdout is expected to be tty, so editor writes control sequences on
stdout, and we have expected behaviour. But we can do better – use
stderr descriptor as tty, and save stdout for piping.